This PR converts the syntax highlighters to plugins. Previously, the
highlighters were part of the Language plugin, using a library shared by
the C-like languages. The Highlighters already had a plugin-like design,
with a clang and default highlighter. This PR takes them out of the
language plugin and into their own highlighter plugin. They are still
accessed thought he HighlightManager.
This change is motivated by #170250. It will allow us to have both a
clang and tree-sitter based highlighter, as well as make it possible to
have a highlighter for a language that doesn't have an upstream language
plugin, like Swift or Rust.
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